This invention relates to apparatus for the aerobic cultivation of micro-organisms, such as bacteria, yeasts, algae, fungi, etc.
The aerobic cultivation of such micro-organisms is becoming of increasingly greater importance, and is being carried out on an ever larger scale. In many instances of such cultivation, the metabolic and growth processes which take place during the cultivation are strongly exothermic, i.e. they take place under liberation of substantial amounts of heat. This heat must be removed, because otherwise the temperature prevailing in the equipment which is used to carry out the cultivation, would rise to a level at which it could inhibit the growth of the micro-organisms or at which it might even destroy the latter.
For example, depending upon whether the fermentation medium used for the growth of yeasts is a carbohydrate, an alcohol, an acid or a hydrocarbon, the heat liberated per kilogram of yeast produced by cultivation amounts to between 3-9000 kcal.
The removal of this liberated heat presents a problem, especially when very rapid fermentation at high production rates is desired, as in the cultivation of single cell proteins (S.C.P.) The amount of heat generated under these circumstances can be so high as to require very highly efficient cooling of the cultivating medium. Thus, in the cultivation of nutrient yeasts, which at the present state of the art can be carried out at generating times of 1.5 to 2.5 hours, several million kcal. of heat must be removed each hour from a fermentation vessel having a capacity of 200-300 cubic meters of fermentation medium.
The fermentation temperature, at which the micro-organisms will properly grow under optimum conditions, is relatively low, being on the order of 30.degree.-40.degree. C. Since the amount of cooling medium (usually water) that is available is frequently restricted, and since the available water is frequently not very cold, it is necessary to incorporate very efficient heat-exchange arrangements in the cultivating equipment, in order to be able to remove the large amounts of undesired heat despite the aforementioned difficulties.
The prior art contains various proposals for dealing with this problem; however, experience has shown that none of these are as effective as is desired.